Phototube



Nov. 13, 1934. s z 1,980,702

PHOTOTUBE Filed Sept. 27, 1933 INVENTOR BERNARD SALZBERGV ATTORNEY Patented Nov. 13, 1934 UNITED STATES rno'ro'rmm Bernard Salzberg, New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 27, 1933, Serial No. 691,117 5 Claims. (01. 250-475) This invention relates to phototubes and more particularly to phototubes in which secondary emission is used to amplify the cathode current.

An object of this invention is to provide an improved vacuum type phototube in which secondary emission from a secondary emitter cathode is utilized to effect amplification of the primary emission from the photocathode.

In a well known type of phototube the primary emission from the photocathode is amplified by due to gas ionization, than the vacuum type tubes.

In accordance with the present invention amplification free from the above mentioned char acteristics of a gas filled tube is obtained with a vacuum type phototube in which amplification of the primary emission current is efiected by utilizing the secondary emission from a large surface electrode, hereinafter called the secondary emitter cathode;

In a preferred embodiment of the present invention the phototube employs a semi-cylindrical shaped photocathode and a flat plate secondary emitter cathode positioned in such a way as to efiect the least possible amount of interference with the light incident upon the photocathode. A grid-like electrode, hereinafter called the collector or accelerator, surrounds the secondary emitter cathode and serves to collect the secondary electrons emitted from the latter cathode and also to accelerate the primary electrons emitted from the photocathode.

A more complete understanding of the invention may be had by reference to'the following description taken in conjunction with the accompanying drawing in which:

Figure 1 is a perspective view, with parts broken away, of a phototube made in accordance with the invention;

Figure 2 is a diagrammatic representation of the phototube and associated circuit;

Figure 3 is a diagrammatic representation of a modified form of phototube and its associated circuit; 1

Figure 4 is another modified phototube structure and its associated circuit. I

The preferred form of phototube illustrated in Figure 1 as embodying one form of my invention is of the vacuum type and comprises a sealed evacuated bulb 10 enclosing the tube electrodes and having the usual base 11 and a reentrant stem 12 which carries an electrode assembly consisting of a photocathode and two cooperating electrodes, one of which is asecondary emitter cathode. The photocathode 13 shown in Figure 1 is a sheet ofmetal, preferably curved or semicylindrical. and mounted on the stem on lead wires 14 in the usual way. The photocathode has on its concave side a light sensitive surface, such as the usual caesium oxide light sensitive surface. One way of preparing such a surface on silver is described in U. S. patent to Bainbridge, #1,901,57'7, March 14, 1933.

The first cooperating electrode or secondary emitter cathode 15 is preferably a fiat rectangular sheet of metal, such as silvered copperor polished nickel, set edgewise to the photocathode 13, and preferably lying in the diametrical plane with respect to which the semi-cylindrical cathode 13 is symmetrically disposed, so that the maximum amount of light may fall on the photocathode 13. The secondary emitter cathode 15 is supported at one edge by a stem lead 16 and is steadied at the other edge by a brace comprising an extension 17 tied through an insulating bead spacer 18 to one of the leads 14 of the photocathode. The secondary emitter cathode may to advantage and for reasons which will be explained have a light sensitive surface.

Thesecond cooperating electrode or anode is a grid-like electrode 19 interposed between the photocathode 13 and the secondary emitter cathode 15. This anode may be similar to the regular flat grids used in radio receiving tubes, and may surround and be coaxial with the secondary emitter cathode. This grid shaped anode or collector electrode 19 is supported at the bottom by a press lead 20 and is steadied by being tied at the top to a lead 14 of the photocathode by an insulating bead spacer 21.

The cesium may be introduced into the tube during manufacture in various well known ways, one convenient way being to evolve cesium vapor by heating a getter cup or holder 22 containing we a reducible salt of cesium mixed with a. reducing manufacture of the phototube of the present invention are similar to those used in the manufacture of the well known thin film caesium oxide types of phototube. In the present tube the secondary emitter cathode surface may be prepared in the same manner as the photocathode surface, the object being to reduce the work function of the surface and thereby permit a copious secondary electron emission from the secondary emitter cathode to the collector electrode. I

In the preferred embodiment described above 5 as well as in all of the modified. forms described hereinafter the principle of amplification of the photoelectric currents is the same. In ordinary operation the secondary emitter cathode is connected to a positive terminal of a source of direct sition with respect to the other-electrodes.

. cathode.

potential and the collector electrode connected to a higher positive potential, the photocathode being connected to the negative side of the-voltage source or sources. During operation of thephototube connected as described above, the electrons which are emitted from the photocathode dueto the action of the incident light are drawn towards the positively charged. accelerator and secondary emitter electrodes. Some ofthese electrons are intercepted by the accelerator electrode but the majority continue on through the interstices of this electrodeto the secondary emitter The magnitude of the photoelectron current intercepted by the accelerator electrode as well as the efliciency of this electrode as a collector-depends upon its configuration and dispo- The ' impactof these primary electrons upon the secondary emitter cathode causes an emission of secondary electrons from its surface. Such sec-- ondary electrons are collected by the more highly positive collector electrode. The amountof amplification effected by this secondary or auxiliary 1 electron emission depends upon the ratio of the secondary electrons, thus emitted, to the primary electrons. If the number of secondary electrons per primary electron is greater than unity, an amplification has taken place. One of the tubes made and operated in accordance with this invention gave an amplification of 4.2 with the secondary emitter cathode operating at 180 volts and the collector electrode operatingat 240 volts. These operating conditions are only representative, however, for the tube may be operated over a range of voltages between 70 and 360 volts for the secondary emitter electrode and between 100 and .450 volts for the collector electrode. The

I; amplification obtained over this range varies be- 1 cathode and the anode are positive with reference to the photocathode 13, the anode being at a higher positive potential with respect to the cathode 13 than the secondary emitter cathode 15. The output or work circuit 25 is connected between the secondary emitter cathode 15 and the collector electrode 19. When the photocathode 13 is illuminated it emits electrons whicli in turn cause an emission of secondary electrons from the secondary emitter cathode 15 in the manner explained above. These secondary electrons being collected by the collector electrode 19 cause a flow of current in the circuit between the electrode 19 and the photocathode 13. This current flowing through the resistance of the work circuit 25 builds up across this resistance a voltage which may be utilized in the output circult 25 in any desirable manner.

Figure 3 shows diagrammatically a modifica tion which has instead of the plate type of seccathode 26 and a'helically wound grid electrode or accelerator 27. The photocathode 13- and the.

, cooperating electrodes 26. and 27 are connected to the battery 23 in the same way as the corre-. sponding electrodes in Figure 2, and the mode of operation is the same as that of the'tube shown in Figure 2. It wasfound that the amplification obtained by this type of tube was not as great as that obtained with the type shown in Figures 1 and 2, as the secondary emitter cathode surface is' smaller.

Figure 4 shows diagrammatically another modification for utilizing secondary emission to increase the output. The secondary emitter cathode is a curved, highly polished solid plate 28, the anode or collector electrode is a curved grid electrode 29, and the photocathode is a curved mesh electrode 30 with its surface made light sen-' sitive. The connections to the battery 23 and work circuit 25 are the same as in Figure 2. In the operation of this type of tube the light falls on the mesh 'photocathode 30, some being intercepted and some passing through the mesh cathode. The light which passes through the cathode 30 and the grid 29 strikes the polished sheet metal secondary emitter cathode 28 and is reflected back to the mesh photocathode 30, thus enhancing the photoelectron emission from the photocathode. The secondary emitter cathode 28emits secondary electrons in the same manner as the secondary emitter cathodes 15 and 26 in Figures 2 and 3 respectively and the voltage is built up in the output circuit 25 as described above in connection with Figure 2. In any of the different types of constructions 1 described above secondary emission may be ob- E, tained from either the collector or the secondary emitter cathode by suitable adjustment of the voltages applied to these elements. The electrode which is to serve as the collector should be always maintained at a higher potential than the secondary emitter cathode.

While a general description and a few specific examples of my invention have been described and illustrated it is to be understood that many .115

other modifications are contemplated, all of which come within the scope of the appended claims.

-What is claimed is:

1. A phototube comprising a light sensitive photocathode, a solid plate-secondary emitter electrode positioned to receive photoelectrons emitted from said photocathode. and a grid-like collector electrode surrounding said secondary emitter electrode.

2. A phototube for-utilizing secondary emission 2 to produce primary emission'amplification comprising a light sensitive primary electron emitting photocathode. a secondary electron emitting cathode, and a collector electrode surrounding said secondary emitter cathode for collecting said primary and secondary electrons.

3. A phototube comprising. a light sensitive photocathode, a solid rectangular light sensitive secondary emitter electrode, and a collector electrode positioned around said secondary emitter electrode, said secondary emitter electrode and 1 collector electrode being angularly disposed to said photocathode.

4. A phototube comprising; a semi-cylindrical shaped light sensitive photoc athode, a thin flat light sensitive secondary emitter cathode, and a flat grid-like collector electrode positioned around said secondary emitter cathode.

5. A phototube having a semi-cylindrical light sensitive photocathode, a solid flat light sensitive secondary emitter'cathode, and a grid-like collector electrode surrounding said secondary emitter cathode.

SALZBERG. 

